Sample pretreatment method for determination of aminocaproic acid in rearranged liquid by high performance liquid chromatography

Abstract

This invention relates to the field of chemical analysis technology and discloses a sample pretreatment method for determining aminocaproic acid in rearranged solutions using high-performance liquid chromatography (HPLC). Specifically, ammonia is first added to the rearranged solution and mixed to adjust the pH to 7-8. Acetonitrile aqueous solution is then added to the resulting mixture, and after thorough mixing, the mixture is allowed to stand to precipitate a solid phase. Finally, the mixture is filtered, and the filtrate is used as the pretreated sample solution. This pretreatment method effectively solves problems such as poor target peak separation, high baseline noise, and column clogging caused by high-salt systems, significantly extending the column life and providing a reliable pretreatment process for the accurate determination of aminocaproic acid content in rearranged solutions.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis technology, specifically to a sample pretreatment method for determining aminocaproic acid in rearranged solutions using high performance liquid chromatography. Background Technology

[0002] Aminocaproic acid is a key impurity and hydrolysis precursor of caprolactam. Its content directly determines the purity, stability, and polymerization performance of caprolactam, and ultimately affects the quality of nylon 6 chips. In the rearrangement solution, aminocaproic acid is an important intermediate in the synthesis of caprolactam, and its content directly affects the yield of the rearrangement reaction. Excessive aminocaproic acid content indicates incomplete Beckmann rearrangement and insufficient cyclohexanone oxime conversion, which reduces the single-pass yield of caprolactam, increases raw material consumption, raises salt content, and affects the stability of the reaction system.

[0003] Due to its high polarity and boiling point, aminocaproic acid in the rearrangement solution is difficult to completely remove by conventional distillation. It tends to accumulate and coke in the reboiler, reducing distillation efficiency, increasing the risk of equipment blockage, and raising refining costs. High levels of aminocaproic acid directly disrupt polymerization kinetics, leading to lower polymer molecular weight, wider distribution, increased yellowness of chips, increased extractables, and negatively impacting spinning performance, causing problems such as yarn breakage, fuzzy fibers, and uneven dyeing. Therefore, accurately determining the aminocaproic acid content in the rearrangement solution is crucial for determining the rearrangement reaction depth, optimizing process parameters, and guiding refining adjustments.

[0004] Traditional methods for detecting rearranged solutions typically involve direct injection or simple filtration followed by injection, which have significant drawbacks: high concentrations of sulfate can easily precipitate and clog the column bed; the salt effect leads to poor target peak resolution, baseline drift, high noise, and retention time shift; and acidic systems can easily trigger caprolactam hydrolysis, resulting in overly high detection results, failing to meet the requirements for accurate quantification and long-term stable detection. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a sample pretreatment method for determining aminocaproic acid in rearranged solutions using high-performance liquid chromatography (HPLC). This method effectively solves problems such as poor target peak separation, high baseline noise, and column clogging caused by high-salt systems, significantly extends the life of the chromatographic column, and provides a reliable pretreatment process for the accurate determination of aminocaproic acid content in rearranged solutions.

[0006] The technical solution of this invention is a sample pretreatment method for determining aminocaproic acid in rearranged solutions by high performance liquid chromatography, comprising the following steps: S1. Add ammonia to the rearrangement solution and mix to adjust the pH to 7-8; Add acetonitrile aqueous solution to the mixture obtained from S2 and S1, mix well, and let stand to precipitate solid phase. S3. Filter the liquid obtained in S2, and use the filtrate as the pretreated sample solution.

[0007] Furthermore, the rearrangement solution is the feed solution after the Beckmann rearrangement reaction of cyclohexanone oxime and fuming sulfuric acid at high temperature; ammonia water is added dropwise.

[0008] Furthermore, the ammonia water is concentrated ammonia water.

[0009] Furthermore, the acetonitrile aqueous solution in S2 is prepared by mixing acetonitrile and water in a volume ratio of 7:3 to 8:2.

[0010] Furthermore, the acetonitrile aqueous solution in S2 is prepared by mixing acetonitrile and water at a volume ratio of 7:3.

[0011] Furthermore, the amount of acetonitrile aqueous solution added is 3 to 4 times the mass of the mixture obtained in S1.

[0012] Furthermore, S2 is left to stand at room temperature for 30-40 minutes. Furthermore, centrifugal filtration is used in S3, with a speed of 8000-10000 r / min and a centrifugation time of 5-10 minutes. The clear liquid after centrifugation is allowed to stand for 3-5 minutes before use.

[0013] Furthermore, the clarified solution was filtered through a 0.22 μm organic phase needle filter membrane, and the initial filtrate was discarded to obtain the test solution for high performance liquid chromatography analysis.

[0014] This invention also relates to the application of the aforementioned sample pretreatment method in high-performance liquid chromatography (HPLC) for the detection of aminocaproic acid content in rearranged solutions.

[0015] The present invention has the following beneficial effects: In this invention, when treating the rearrangement solution, ammonia is first added for neutralization to adjust the system to neutral or weakly alkaline, avoiding the loss of aminocaproic acid isomers, inhibiting the acidic hydrolysis of caprolactam, preventing overestimation of detection results, and protecting the chromatographic column from acid corrosion. Then, acetonitrile aqueous solution is added for mixing. While aminocaproic acid has high solubility, the solubility of ammonium sulfate produced during neutralization decreases sharply, causing it to co-precipitate with large molecular colloids and impurities in the rearrangement solution, achieving efficient desalting and impurity removal. The treated solution is filtered to remove suspended particles and precipitates, resulting in a filtrate free of sticky impurities. This filtrate is used for high-performance liquid chromatography (HPLC) detection, avoiding column head clogging and packing material contamination, and significantly extending column life.

[0016] After sample pretreatment using the method of this invention, the target peak of aminocaproic acid in the high-performance liquid chromatography (HPLC) chromatogram shows good separation, symmetrical peak shape, stable baseline, and stable retention time, resulting in accurate and reliable quantification. Attached Figure Description

[0017] Figure 1 This is a photograph of the salting-out process in Example 1; Figure 2 This is the standard curve graph from Example 1; Figure 3 This is the liquid chromatogram of the sample to be tested in Example 1; Figure 4 The liquid chromatogram for acetonitrile:pure water = 8:2 in Example 2; Figure 5 The liquid chromatogram of acetonitrile:pure water = 9:1 in Example 3; Figure 6 The liquid chromatogram of acetonitrile:pure water = 6:4 in Example 4; Figure 7 This is the liquid chromatogram of pure acetonitrile from Example 5; Figure 8 This is a photograph of acetonitrile water added directly without neutralization in Example 6. Detailed Implementation

[0018] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all raw materials and reagents used are commercially available.

[0019] The rearrangement solutions used in the following examples and comparative examples are the feed solutions after the Beckmann rearrangement reaction of cyclohexanone oxime and fuming sulfuric acid at high temperature (80~120℃). The main component is caprolactam, and the impurities are mainly sulfates, aminocaproic acid, cyclohexanone, enones, octahydrophenazine, etc.

[0020] This invention provides a sample pretreatment method for determining aminocaproic acid in rearranged solutions using high-performance liquid chromatography, comprising the following steps: S1. Add ammonia water to the rearrangement solution and mix. It is preferable to add the ammonia water dropwise while mixing, and adjust the pH of the whole system to 7~8. Add acetonitrile aqueous solution to the mixture obtained from S2 and S1, mix well, and let stand to precipitate solid phase. S3. Filter the liquid obtained in S2, and use the filtrate as the pretreated sample solution.

[0021] In some embodiments, the acetonitrile aqueous solution is prepared by mixing acetonitrile and water at a volume ratio of 7:3 to 8:2. A 7:3 ratio is preferred. The amount of acetonitrile aqueous solution added is 3 to 4 times the mass of the mixture obtained in S1. The precipitation is allowed to occur at room temperature for 30-40 minutes. Concentrated ammonia is used.

[0022] In some embodiments, centrifugal filtration is preferably used in S3, with a speed of 10000 r / min for 10 minutes. The supernatant after centrifugation is allowed to stand for 5 minutes before use. Before entering the chromatographic column, the supernatant is filtered through a 0.22 μm organic phase needle filter membrane, and the initial filtrate is discarded to obtain the test solution for high performance liquid chromatography analysis.

[0023] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention.

[0024] Example 1 S1. Take 2.0623g of the refluxing solution and add ammonia water dropwise while stirring. Adjust the pH of the entire system to 7. The mixture obtained from S2 and S1 was supplemented with 6.1273 g of acetonitrile aqueous solution, which was prepared by mixing acetonitrile and ultrapure water at a volume ratio of 7:3. After addition, the mixture was thoroughly mixed and allowed to stand for 5 minutes. A white precipitate formed at the bottom of the container. Figure 1 As shown; S3. Filter the liquid obtained in S2 using a high-speed centrifuge at 10,000 r / min for 10 minutes, and let it stand for about 5 minutes to achieve complete separation of the solid phase precipitate and the liquid phase sample.

[0025] S4. High-performance liquid chromatography (HPLC) detection: Aspirate the above sample with a disposable syringe, filter it through a 0.22μm organic phase needle filter membrane, discard the initial filtrate, and obtain the test solution for HPLC analysis. Inject the sample for detection.

[0026] Chromatographic conditions: Thermo Fisher Scientific Vanquish 3000 HPLC; Wavelength: 200 nm; Column flow rate: 0.4 mL / min; Mobile phase: Acetonitrile:buffered saline = 85:15 V / V; Column temperature: 40°C; Detector: Diode array detector; Quantitative loop: 20 μL; Column: Aminocaproic acid-specific column. Phosphate buffer preparation: 0.3403 g of analytical grade potassium dihydrogen phosphate (KH₂PO₄) and 0.8954 g of analytical grade disodium hydrogen phosphate (Na₂HPO₄·12H₂O) were dissolved in carbon dioxide-free water, transferred to a 500 mL volumetric flask, and diluted to the mark with carbon dioxide-free water.

[0027] Preparation of standard solutions: Dissolve 0.2002 g of aminocaproic acid in a 100 ml volumetric flask to prepare a stock solution with a concentration of 2002 mg / L. Take 0.1 ml of this stock solution and dissolve it in a 10 ml volumetric flask (concentration 20.02 mg / L, this is standard sample 1); take 0.2 ml of this stock solution and dissolve it in a 10 ml volumetric flask (concentration 40.04 mg / L, this is standard sample 2); take 1 ml of this stock solution and dissolve it in a 25 ml volumetric flask (concentration 80.08 mg / L, this is standard sample 3); take 0.5 ml of this stock solution and dissolve it in a 10 ml volumetric flask (concentration 100.1 mg / L, this is standard sample 4); take 1 ml of this stock solution and dissolve it in a 10 ml volumetric flask (concentration 200.2 mg / L, this is standard sample 5). Dilute each sample to the mark with 80% acetonitrile solution. Analyze the obtained standard solutions and plot the concentration-peak area standard curve as shown below. Figure 2 The standard curve equation is: y = 0.360x - 1.315, R0 2 =0.999, indicating good relevance.

[0028] The test solution in S4 was used for testing, and the retention time of aminocaproic acid was 9.665 min. Figure 3 As shown, the peak shape is symmetrical, without tailing, and the baseline is completely separated and stable without fluctuation. The RSD of five consecutive injections is <0.5%, the background concentration is 48.2244 mg / L, which translates to 143.2795 mg / L of aminocaproic acid in the original rearrangement. The spiked recovery rate is 98.5-101.2%, which meets the industrial analysis standards. The results are shown in Table 3.

[0029] Example 2 The specific operation process is the same as in Example 1, and the same sample is used, the only difference being that the acetonitrile-water ratio is adjusted to 8:2. The sample weight is 2.2569 g, and the total mass of the added acetonitrile-water is 6.3701 g. The final detection data are shown in Table 1, and the sample liquid chromatography spectrum is shown below. Figure 4 .

[0030] Test results: The aminocaproic acid peak shape was good, and the salting-out precipitation was sufficient. The detection concentration was 50.7856 mg / L, and the original redistribution solution was 143.3401 mg / L. The spiked recovery rate was 97.3-100.8%, which was slightly lower than that of the 7:3 ratio, indicating higher consumption and increased detection costs.

[0031] Example 3 The specific operation process is the same as in Example 1, and the same sample is used, the only difference being that the acetonitrile-to-water ratio is adjusted to 9:1. The sample weight is 2.0935g, and the total mass of the added acetonitrile-to-water is 6.5945g. The final detection data are shown in Table 1, and the sample liquid chromatography spectrum is shown below. Figure 5 .

[0032] Test results: The salting-out precipitation rate was accelerated, but the high organic phase ratio caused a small amount of aminocaproic acid to be lost through co-precipitation. The detected concentration was 42.9123 mg / L, and the original redistribution concentration was 135.1700 mg / L. The spiked recovery rate was 94.5-96.5%, the quantitative results were low, and the accuracy did not meet the standards.

[0033] Example 4 The specific operation process is the same as in Example 1, and the same sample is used, the only difference being that the acetonitrile-water ratio is adjusted to 6:4. The sample weight is 2.0423g, and the total mass of the added acetonitrile and water is 6.5792g. The final detection data are shown in Table 1, and the sample liquid chromatography spectrum is shown below. Figure 6 .

[0034] Test results: The solvent had an excessively high water content, resulting in incomplete ammonium sulfate precipitation. A large amount of sulfate remained in the supernatant, and the aminocaproic acid peak was severely tailed, making it impossible to separate from adjacent impurity peaks, which directly led to difficulties in qualitative and quantitative analysis.

[0035] Example 5 The specific operation process is the same as in Example 1, and the same sample is used, the only difference being that acetonitrile water is replaced with pure acetonitrile. The sample weight was 2.1038 g, and the total mass of the acetonitrile water was 6.9020 g. The final detection data are shown in Table 1, and the sample liquid chromatography spectrum is shown below. Figure 7 .

[0036] Test results: In a pure acetonitrile system, aminocaproic acid and sulfate co-precipitated extensively into the solid phase, significantly reducing the target analyte concentration in the liquid phase. The detected concentration was 37.8492 mg / L, compared to 124.1730 mg / L in the original rearrangement; the recovery rate was 76.8-82.2%. Example 6 The specific operation process is the same as in Example 1, and the same sample is used as in Example 1, without neutralization treatment. The sample is strongly acidic, such as... Figure 8 As shown. This sample was not injected due to the strong acid matrix corroding the chromatographic column.

[0037] Table 1. Aminohexanoic acid content at different proportions in the same sample

[0038] Table 2. Aminocaproic acid content of the same sample at different standing times.

[0039] Table 3. Acetonitrile:Water = 7:3 Spiking Recovery Data

[0040] Table 4. Acetonitrile:Water = 8:2 Spiking Recovery Data

[0041] Table 5. Acetonitrile:Water = 9:1 Spiking Recovery Data

[0042] Table 6. Spike recovery data for pure acetonitrile

[0043] As can be seen from the above examples, after processing with the method of the present invention, the aminohexanoic acid chromatographic peak in the chromatogram is sharp and symmetrical, achieving complete baseline separation from the preceding and following impurity peaks; the baseline is stable without drift, and the retention time of the target analyte is stable, providing a basic guarantee for accurate integration. Compared with Example 1, Example 2 shows the precipitation of a white solid, indicating good separation of the aminohexanoic acid peak, but the amount of acetonitrile used is greater than in Example 1, resulting in a slightly higher detection cost. Compared with Example 1, Example 3 shows a faster salting-out precipitation rate, but the organic phase ratio is too high, resulting in the precipitation of a small amount of aminohexanoic acid and a slight decrease in the detection recovery rate. Compared with Example 1, Example 4 shows incomplete salting-out in the sample, with residual sulfate in the upper layer solution, causing severe tailing of the aminohexanoic acid peak, making it impossible to separate from adjacent impurity peaks, directly leading to difficulties in qualitative and quantitative analysis. Compared with Example 1, Example 5 shows a large amount of aminohexanoic acid co-precipitation, resulting in a severely low quantitative result, making it unusable for detection.

[0044] In terms of sample solution stability, the same sample was analyzed after standing for 30 min, 60 min, and 90 min respectively, as shown in Table 2. The acetonitrile-water 7:3 system showed the least fluctuation and the best stability compared to the other examples.

[0045] Regarding the sample spiked recoveries, when acetonitrile-water 7:3, acetonitrile-water 8:2, acetonitrile-water 9:1, and pure acetonitrile systems were spiked with different concentrations of standard samples (see Tables 3, 4, 5, and 6), the recovery rate of the acetonitrile-water 7:3 system consistently ranged from 98.6% to 101.2%, meeting the accuracy requirement of 98%-102% for macro-analysis, with an RSD < 0.5, indicating no significant systematic bias. The recovery rate of the acetonitrile-water 8:2 system (97.28%) was slightly below the lower limit of the standard, while the recoveries of the acetonitrile-water 9:1 and pure acetonitrile systems deviated significantly from 100%, indicating excessive quantitative errors and rendering them unsuitable for quantitative detection.

[0046] The above embodiments describe preferred embodiments of the present invention, but the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other way. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A sample pretreatment method for determining aminocaproic acid in rearranged solutions by high performance liquid chromatography, characterized in that, Includes the following steps: S1. Add ammonia to the rearrangement solution and mix to adjust the pH to 7-8; Add acetonitrile aqueous solution to the mixture obtained from S2 and S1, mix well, and let stand to precipitate solid phase. S3. Filter the liquid obtained in S2, and use the filtrate as the pretreated sample solution.

2. The method according to claim 1, characterized in that: The rearrangement solution is the feed solution after the Beckmann rearrangement reaction of cyclohexanone oxime and fuming sulfuric acid at high temperature; ammonia water is added dropwise.

3. The method according to claim 2, characterized in that: The ammonia solution is concentrated ammonia solution.

4. The method according to claim 1, characterized in that: The acetonitrile aqueous solution in S2 is prepared by mixing acetonitrile and water in a volume ratio of 7:3 to 8:

2.

5. The method according to claim 4, characterized in that: The acetonitrile aqueous solution in S2 is prepared by mixing acetonitrile and water in a volume ratio of 7:

3.

6. The method according to claim 1, characterized in that: The amount of acetonitrile aqueous solution added is 3 to 4 times the mass of the mixture obtained in S1.

7. The method according to claim 1, characterized in that: S2 should be left to stand at room temperature for 30-40 minutes.

8. The method according to claim 1, characterized in that: When filtering in S3, centrifugal filtration is used at a speed of 8000-10000 r / min for 5-10 minutes. After centrifugation, the clear liquid should be allowed to stand for 3-5 minutes before use.

9. The method according to claim 8, characterized in that: The clarified solution was filtered through a 0.22 μm organic phase needle filter membrane, and the initial filtrate was discarded to obtain the test solution for high performance liquid chromatography analysis.

10. The application of the sample pretreatment method according to any one of claims 1 to 9 in the high-performance liquid chromatography (HPLC) detection of aminocaproic acid content in rearranged solutions.

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